Lille wouldn't want to let go of Bastos easily. But French teams are not known for their ability to keep their players. I think if a big team really wanted him, he'll go for something between 7-10 Million Euro's. Dunga btw said that he liked Bastos, and that if he kept playing like this, he would get a callup to the NT.
As for the fullbacks, i do not see any good enough leftback. Jeremy Mathieu of Toulouse was a pretty good leftback. Roma was interested in him last summer, but a leg injury that put him out for 4 months badly affected his season and his form. He's been average lately. Taiwo is viewed as the best leftback in the Ligue, but i don't think he's good enough either. Too clumsy for my liking, he makes basic mistakes that make him look average sometimes. He gets away with them in the Ligue, but he probably won't get away with them in a big league.
As for the rightback position. I believe Dabo of St Etienne will develop into a fine fullback. Very versatile player, he can play leftback as well, and he can play as a defensive midfielder. Bags of pace and stamina, covers a lot of ground and is almost impossible to beat. I think this guy would be a very good signing. He would definitely be a big upgrade over Grygera.